TLS Working Group Y. Nir
Internet-Draft Check Point
Obsoletes: 4492 (if approved) S. Josefsson
Intended status: Standards Track SJD AB
Expires: September 23, 2016 M. Pegourie-Gonnard
Independent / PolarSSL
March 22, 2016
Elliptic Curve Cryptography (ECC) Cipher Suites for Transport LayerSecurity (TLS) Versions 1.2 and Earlierdraft-ietf-tls-rfc4492bis-07
Abstract
This document describes key exchange algorithms based on Elliptic
Curve Cryptography (ECC) for the Transport Layer Security (TLS)
protocol. In particular, it specifies the use of Ephemeral Elliptic
Curve Diffie-Hellman (ECDHE) key agreement in a TLS handshake and the
use of Elliptic Curve Digital Signature Algorithm (ECDSA) and Edwards
Digital Signature Algorithm (EdDSA) as new authentication mechanisms.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 23, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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RSA, ECC offers equivalent security with smaller key sizes. This is
illustrated in the following table, based on [Lenstra_Verheul], which
gives approximate comparable key sizes for symmetric- and asymmetric-
key cryptosystems based on the best-known algorithms for attacking
them.
+-----------+-------+------------+
| Symmetric | ECC | DH/DSA/RSA |
+-----------+-------+------------+
| 80 | >=158 | 1024 |
| 112 | >=221 | 2048 |
| 128 | >=252 | 3072 |
| 192 | >=379 | 7680 |
| 256 | >=506 | 15360 |
+-----------+-------+------------+
Table 1: Comparable Key Sizes (in bits)
Smaller key sizes result in savings for power, memory, bandwidth, and
computational cost that make ECC especially attractive for
constrained environments.
This document describes additions to TLS to support ECC, applicable
to TLS versions 1.0 [RFC2246], 1.1 [RFC4346], and 1.2 [RFC5246]. The
use of ECC in TLS 1.3 is defined in [I-D.ietf-tls-tls13], and is
explicitly out of scope for this document. In particular, this
document defines:
o the use of the Elliptic Curve Diffie-Hellman key agreement scheme
with ephemeral keys to establish the TLS premaster secret, and
o the use of ECDSA certificates for authentication of TLS peers.
The remainder of this document is organized as follows. Section 2
provides an overview of ECC-based key exchange algorithms for TLS.
Section 3 describes the use of ECC certificates for client
authentication. TLS extensions that allow a client to negotiate the
use of specific curves and point formats are presented in Section 4.
Section 5 specifies various data structures needed for an ECC-based
handshake, their encoding in TLS messages, and the processing of
those messages. Section 6 defines ECC-based cipher suites and
identifies a small subset of these as recommended for all
implementations of this specification. Section 7 discusses security
considerations. Section 8 describes IANA considerations for the name
spaces created by this document's predecessor. Section 9 gives
acknowledgements. Appendix B provides differences from [RFC4492],
the document that this one replaces.
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Implementation of this specification requires familiarity with TLS,
TLS extensions [RFC4366], and ECC (TBD: reference Wikipedia here?).
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Key Exchange Algorithm
This document defines three new ECC-based key exchange algorithms for
TLS. All of them use Ephemeral ECDH (ECDHE) to compute the TLS
premaster secret, and they differ only in the mechanism (if any) used
to authenticate them. The derivation of the TLS master secret from
the premaster secret and the subsequent generation of bulk
encryption/MAC keys and initialization vectors is independent of the
key exchange algorithm and not impacted by the introduction of ECC.
Table 2 summarizes the new key exchange algorithms. All of these key
exchange algorithms provide forward secrecy.
+-------------+------------------------------------------------+
| Algorithm | Description |
+-------------+------------------------------------------------+
| ECDHE_ECDSA | Ephemeral ECDH with ECDSA or EdDSA signatures. |
| ECDHE_RSA | Ephemeral ECDH with RSA signatures. |
| ECDH_anon | Anonymous ephemeral ECDH, no signatures. |
+-------------+------------------------------------------------+
Table 2: ECC Key Exchange Algorithms
These key exchanges are analogous to DHE_DSS, DHE_RSA, and DH_anon,
respectively.
With ECDHE_RSA, a server can reuse its existing RSA certificate and
easily comply with a constrained client's elliptic curve preferences
(see Section 4). However, the computational cost incurred by a
server is higher for ECDHE_RSA than for the traditional RSA key
exchange, which does not provide forward secrecy.
The anonymous key exchange algorithm does not provide authentication
of the server or the client. Like other anonymous TLS key exchanges,
it is subject to man-in-the-middle attacks. Implementations of this
algorithm SHOULD provide authentication by other means.
Note that there is no structural difference between ECDH and ECDSA
keys. A certificate issuer may use X.509 v3 keyUsage and
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extendedKeyUsage extensions to restrict the use of an ECC public key
to certain computations. This document refers to an ECC key as ECDH-
capable if its use in ECDH is permitted. ECDSA-capable and EdDSA-
capable are defined similarly.
Client Server
------ ------
ClientHello -------->
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*+
<-------- ServerHelloDone
Certificate*+
ClientKeyExchange
CertificateVerify*+
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
* message is not sent under some conditions
+ message is not sent unless client authentication
is desired
Figure 1: Message flow in a full TLS 1.2 handshake
Figure 1 shows all messages involved in the TLS key establishment
protocol (aka full handshake). The addition of ECC has direct impact
only on the ClientHello, the ServerHello, the server's Certificate
message, the ServerKeyExchange, the ClientKeyExchange, the
CertificateRequest, the client's Certificate message, and the
CertificateVerify. Next, we describe the ECC key exchange algorithm
in greater detail in terms of the content and processing of these
messages. For ease of exposition, we defer discussion of client
authentication and associated messages (identified with a + in
Figure 1) until Section 3 and of the optional ECC-specific extensions
(which impact the Hello messages) until Section 4.
2.1. ECDHE_ECDSA
In ECDHE_ECDSA, the server's certificate MUST contain an ECDSA- or
EdDSA-capable public key.
The server sends its ephemeral ECDH public key and a specification of
the corresponding curve in the ServerKeyExchange message. These
parameters MUST be signed with ECDSA or EdDSA using the private key
corresponding to the public key in the server's Certificate.
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The client generates an ECDH key pair on the same curve as the
server's ephemeral ECDH key and sends its public key in the
ClientKeyExchange message.
Both client and server perform an ECDH operation Section 5.10 and use
the resultant shared secret as the premaster secret.
2.2. ECDHE_RSA
This key exchange algorithm is the same as ECDHE_ECDSA except that
the server's certificate MUST contain an RSA public key authorized
for signing, and that the signature in the ServerKeyExchange message
must be computed with the corresponding RSA private key.
2.3. ECDH_anon
NOTE: Despite the name beginning with "ECDH_" (no E), the key used in
ECDH_anon is ephemeral just like the key in ECDHE_RSA and
ECDHE_ECDSA. The naming follows the example of DH_anon, where the
key is also ephemeral but the name does not reflect it. TBD: Do we
want to rename this so that it makes sense?
In ECDH_anon, the server's Certificate, the CertificateRequest, the
client's Certificate, and the CertificateVerify messages MUST NOT be
sent.
The server MUST send an ephemeral ECDH public key and a specification
of the corresponding curve in the ServerKeyExchange message. These
parameters MUST NOT be signed.
The client generates an ECDH key pair on the same curve as the
server's ephemeral ECDH key and sends its public key in the
ClientKeyExchange message.
Both client and server perform an ECDH operation and use the
resultant shared secret as the premaster secret. All ECDH
calculations are performed as specified in Section 5.10.
This specification does not impose restrictions on signature schemes
used anywhere in the certificate chain. The previous version of this
document required the signatures to match, but this restriction,
originating in previous TLS versions is lifted here as it had been in
RFC 5246.
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This document defines a client authentication mechanism, named after
the type of client certificate involved: ECDSA_sign. The ECDSA_sign
mechanism is usable with any of the non-anonymous ECC key exchange
algorithms described in Section 2 as well as other non-anonymous
(non-ECC) key exchange algorithms defined in TLS.
The server can request ECC-based client authentication by including
this certificate type in its CertificateRequest message. The client
must check if it possesses a certificate appropriate for the method
suggested by the server and is willing to use it for authentication.
If these conditions are not met, the client should send a client
Certificate message containing no certificates. In this case, the
ClientKeyExchange should be sent as described in Section 2, and the
CertificateVerify should not be sent. If the server requires client
authentication, it may respond with a fatal handshake failure alert.
If the client has an appropriate certificate and is willing to use it
for authentication, it must send that certificate in the client's
Certificate message (as per Section 5.6) and prove possession of the
private key corresponding to the certified key. The process of
determining an appropriate certificate and proving possession is
different for each authentication mechanism and described below.
NOTE: It is permissible for a server to request (and the client to
send) a client certificate of a different type than the server
certificate.
3.1. ECDSA_sign
To use this authentication mechanism, the client MUST possess a
certificate containing an ECDSA- or EdDSA-capable public key.
The client proves possession of the private key corresponding to the
certified key by including a signature in the CertificateVerify
message as described in Section 5.8.
4. TLS Extensions for ECC
Two new TLS extensions are defined in this specification: (i) the
Supported Elliptic Curves Extension, and (ii) the Supported Point
Formats Extension. These allow negotiating the use of specific
curves and point formats (e.g., compressed vs. uncompressed,
respectively) during a handshake starting a new session. These
extensions are especially relevant for constrained clients that may
only support a limited number of curves or point formats. They
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follow the general approach outlined in [RFC4366]; message details
are specified in Section 5. The client enumerates the curves it
supports and the point formats it can parse by including the
appropriate extensions in its ClientHello message. The server
similarly enumerates the point formats it can parse by including an
extension in its ServerHello message.
A TLS client that proposes ECC cipher suites in its ClientHello
message SHOULD include these extensions. Servers implementing ECC
cipher suites MUST support these extensions, and when a client uses
these extensions, servers MUST NOT negotiate the use of an ECC cipher
suite unless they can complete the handshake while respecting the
choice of curves and compression techniques specified by the client.
This eliminates the possibility that a negotiated ECC handshake will
be subsequently aborted due to a client's inability to deal with the
server's EC key.
The client MUST NOT include these extensions in the ClientHello
message if it does not propose any ECC cipher suites. A client that
proposes ECC cipher suites may choose not to include these
extensions. In this case, the server is free to choose any one of
the elliptic curves or point formats listed in Section 5. That
section also describes the structure and processing of these
extensions in greater detail.
In the case of session resumption, the server simply ignores the
Supported Elliptic Curves Extension and the Supported Point Formats
Extension appearing in the current ClientHello message. These
extensions only play a role during handshakes negotiating a new
session.
5. Data Structures and Computations
This section specifies the data structures and computations used by
ECC-based key mechanisms specified in the previous three sections.
The presentation language used here is the same as that used in TLS.
Since this specification extends TLS, these descriptions should be
merged with those in the TLS specification and any others that extend
TLS. This means that enum types may not specify all possible values,
and structures with multiple formats chosen with a select() clause
may not indicate all possible cases.
5.1. Client Hello Extensions
This section specifies two TLS extensions that can be included with
the ClientHello message as described in [RFC4366], the Supported
Elliptic Curves Extension and the Supported Point Formats Extension.
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When these extensions are sent:
The extensions SHOULD be sent along with any ClientHello message that
proposes ECC cipher suites.
Meaning of these extensions:
These extensions allow a client to enumerate the elliptic curves it
supports and/or the point formats it can parse.
Structure of these extensions:
The general structure of TLS extensions is described in [RFC4366],
and this specification adds two new types to ExtensionType.
enum {
elliptic_curves(10),
ec_point_formats(11)
} ExtensionType;
elliptic_curves (Supported Elliptic Curves Extension): Indicates the
set of elliptic curves supported by the client. For this
extension, the opaque extension_data field contains
EllipticCurveList. See Section 5.1.1 for details.
ec_point_formats (Supported Point Formats Extension): Indicates the
set of point formats that the client can parse. For this
extension, the opaque extension_data field contains
ECPointFormatList. See Section 5.1.2 for details.
Actions of the sender:
A client that proposes ECC cipher suites in its ClientHello message
appends these extensions (along with any others), enumerating the
curves it supports and the point formats it can parse. Clients
SHOULD send both the Supported Elliptic Curves Extension and the
Supported Point Formats Extension. If the Supported Point Formats
Extension is indeed sent, it MUST contain the value 0 (uncompressed)
as one of the items in the list of point formats.
Actions of the receiver:
A server that receives a ClientHello containing one or both of these
extensions MUST use the client's enumerated capabilities to guide its
selection of an appropriate cipher suite. One of the proposed ECC
cipher suites must be negotiated only if the server can successfully
complete the handshake while using the curves and point formats
supported by the client (cf. Section 5.3 and Section 5.4).
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NOTE: A server participating in an ECDHE_ECDSA key exchange may use
different curves for the ECDSA or EdDSA key in its certificate, and
for the ephemeral ECDH key in the ServerKeyExchange message. The
server MUST consider the extensions in both cases.
If a server does not understand the Supported Elliptic Curves
Extension, does not understand the Supported Point Formats Extension,
or is unable to complete the ECC handshake while restricting itself
to the enumerated curves and point formats, it MUST NOT negotiate the
use of an ECC cipher suite. Depending on what other cipher suites
are proposed by the client and supported by the server, this may
result in a fatal handshake failure alert due to the lack of common
cipher suites.
5.1.1. Supported Elliptic Curves ExtensionRFC 4492 defined 25 different curves in the NamedCurve registry (now
renamed the "Supported Groups" registry, although the enumeration
below is still named NamedCurve) for use in TLS. Only three have
seen much use. This specification is deprecating the rest (with
numbers 1-22). This specification also deprecates the explicit
curves with identifiers 0xFF01 and 0xFF02. It also adds the new
curves defined in [RFC7748] and [CFRG-EdDSA]. The end result is as
follows:
enum {
deprecated(1..22),
secp256r1 (23), secp384r1 (24), secp521r1 (25),
ecdh_x25519(29), ecdh_x448(30),
eddsa_ed25519(TBD3), eddsa_ed448(TBD4),
reserved (0xFE00..0xFEFF),
deprecated(0xFF01..0xFF02),
(0xFFFF)
} NamedCurve;
Note that other specification have since added other values to this
enumeration.
secp256r1, etc: Indicates support of the corresponding named curve or
class of explicitly defined curves. The named curves secp256r1,
secp384r1, and secp521r1 are specified in SEC 2 [SECG-SEC2]. These
curves are also recommended in ANSI X9.62 [ANSI.X9-62.2005] and FIPS
186-4 [FIPS.186-4]. ecdh_x25519 and ecdh_x448 are defined in
[RFC7748]. eddsa_ed25519 and eddsa_ed448 are signature-only curves
defined in [CFRG-EdDSA]. Values 0xFE00 through 0xFEFF are reserved
for private use.
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The NamedCurve name space is maintained by IANA. See Section 8 for
information on how new value assignments are added.
struct {
NamedCurve elliptic_curve_list<2..2^16-1>
} EllipticCurveList;
Items in elliptic_curve_list are ordered according to the client's
preferences (favorite choice first).
As an example, a client that only supports secp256r1 (aka NIST P-256;
value 23 = 0x0017) and secp384r1 (aka NIST P-384; value 24 = 0x0018)
and prefers to use secp256r1 would include a TLS extension consisting
of the following octets. Note that the first two octets indicate the
extension type (Supported Elliptic Curves Extension):
00 0A 00 06 00 04 00 17 00 18
5.1.2. Supported Point Formats Extension
enum {
uncompressed (0),
ansiX962_compressed_prime (1),
ansiX962_compressed_char2 (2),
reserved (248..255)
} ECPointFormat;
struct {
ECPointFormat ec_point_format_list<1..2^8-1>
} ECPointFormatList;
Three point formats were included in the definition of ECPointFormat
above. This specification deprecates all but the uncompressed point
format. Implementations of this document MUST support the
uncompressed format for all of their supported curves, and MUST NOT
support other formats for curves defined in this specification. For
backwards compatibility purposes, the point format list extension
MUST still be included, and contain exactly one value: the
uncompressed point format (0).
The ECPointFormat name space is maintained by IANA. See Section 8
for information on how new value assignments are added.
Items in ec_point_format_list are ordered according to the client's
preferences (favorite choice first).
A client compliant with this specification that supports no other
curves MUST send the following octets; note that the first two octets
indicate the extension type (Supported Point Formats Extension):
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00 0B 00 02 01 00
5.2. Server Hello Extension
This section specifies a TLS extension that can be included with the
ServerHello message as described in [RFC4366], the Supported Point
Formats Extension.
When this extension is sent:
The Supported Point Formats Extension is included in a ServerHello
message in response to a ClientHello message containing the Supported
Point Formats Extension when negotiating an ECC cipher suite.
Meaning of this extension:
This extension allows a server to enumerate the point formats it can
parse (for the curve that will appear in its ServerKeyExchange
message when using the ECDHE_ECDSA, ECDHE_RSA, or ECDH_anon key
exchange algorithm.
Structure of this extension:
The server's Supported Point Formats Extension has the same structure
as the client's Supported Point Formats Extension (see
Section 5.1.2). Items in ec_point_format_list here are ordered
according to the server's preference (favorite choice first). Note
that the server may include items that were not found in the client's
list (e.g., the server may prefer to receive points in compressed
format even when a client cannot parse this format: the same client
may nevertheless be capable of outputting points in compressed
format).
Actions of the sender:
A server that selects an ECC cipher suite in response to a
ClientHello message including a Supported Point Formats Extension
appends this extension (along with others) to its ServerHello
message, enumerating the point formats it can parse. The Supported
Point Formats Extension, when used, MUST contain the value 0
(uncompressed) as one of the items in the list of point formats.
Actions of the receiver:
A client that receives a ServerHello message containing a Supported
Point Formats Extension MUST respect the server's choice of point
formats during the handshake (cf. Section 5.6 and Section 5.7). If
no Supported Point Formats Extension is received with the
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ServerHello, this is equivalent to an extension allowing only the
uncompressed point format.
5.3. Server Certificate
When this message is sent:
This message is sent in all non-anonymous ECC-based key exchange
algorithms.
Meaning of this message:
This message is used to authentically convey the server's static
public key to the client. The following table shows the server
certificate type appropriate for each key exchange algorithm. ECC
public keys MUST be encoded in certificates as described in
Section 5.9.
NOTE: The server's Certificate message is capable of carrying a chain
of certificates. The restrictions mentioned in Table 3 apply only to
the server's certificate (first in the chain).
+-------------+-----------------------------------------------------+
| Algorithm | Server Certificate Type |
+-------------+-----------------------------------------------------+
| ECDHE_ECDSA | Certificate MUST contain an ECDSA- or EdDSA-capable |
| | public key. |
| ECDHE_RSA | Certificate MUST contain an RSA public key |
| | authorized for use in digital signatures. |
+-------------+-----------------------------------------------------+
Table 3: Server Certificate Types
Structure of this message:
Identical to the TLS Certificate format.
Actions of the sender:
The server constructs an appropriate certificate chain and conveys it
to the client in the Certificate message. If the client has used a
Supported Elliptic Curves Extension, the public key in the server's
certificate MUST respect the client's choice of elliptic curves; in
particular, the public key MUST employ a named curve (not the same
curve as an explicit curve) unless the client has indicated support
for explicit curves of the appropriate type. If the client has used
a Supported Point Formats Extension, both the server's public key
point and (in the case of an explicit curve) the curve's base point
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MUST respect the client's choice of point formats. (A server that
cannot satisfy these requirements MUST NOT choose an ECC cipher suite
in its ServerHello message.)
Actions of the receiver:
The client validates the certificate chain, extracts the server's
public key, and checks that the key type is appropriate for the
negotiated key exchange algorithm. (A possible reason for a fatal
handshake failure is that the client's capabilities for handling
elliptic curves and point formats are exceeded; cf. Section 5.1.)
5.4. Server Key Exchange
When this message is sent:
This message is sent when using the ECDHE_ECDSA, ECDHE_RSA, and
ECDH_anon key exchange algorithms.
Meaning of this message:
This message is used to convey the server's ephemeral ECDH public key
(and the corresponding elliptic curve domain parameters) to the
client.
The ECCCurveType enum used to have values for explicit prime and for
explicit char2 curves. Those values are now deprecated, so only one
value remains:
Structure of this message:
enum {
deprecated (1..2),
named_curve (3),
reserved(248..255)
} ECCurveType;
The value named_curve indicates that a named curve is used. This
option SHOULD be used when applicable.
Values 248 through 255 are reserved for private use.
The ECCurveType name space is maintained by IANA. See Section 8 for
information on how new value assignments are added.
RFC 4492 had a specification for an ECCurve structure and an
ECBasisType structure. Both of these are omitted now because they
were only used with the now deprecated explicit curves.
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struct {
opaque point <1..2^8-1>;
} ECPoint;
This is the byte string representation of an elliptic curve point
following the conversion routine in Section 4.3.6 of
[ANSI.X9-62.2005]. This byte string may represent an elliptic curve
point in uncompressed or compressed format; it MUST conform to what
the client has requested through a Supported Point Formats Extension
if this extension was used. For the X25519 and X448 curves, the only
valid representation is the one specified in [RFC7748] - a 32- or
56-octet representation of the u value of the point. This structure
MUST NOT be used with Ed25519 and Ed448 public keys.
struct {
ECCurveType curve_type;
select (curve_type) {
case named_curve:
NamedCurve namedcurve;
};
} ECParameters;
This identifies the type of the elliptic curve domain parameters.
Specifies a recommended set of elliptic curve domain parameters. All
those values of NamedCurve are allowed that refer to a curve capable
of Diffie-Hellman. With the deprecation of the explicit curves, this
now includes all values of NamedCurve except eddsa_ed25519(TBD3) and
eddsa_ed448(TBD4).
struct {
ECParameters curve_params;
ECPoint public;
} ServerECDHParams;
Specifies the elliptic curve domain parameters associated with the
ECDH public key.
The ephemeral ECDH public key.
The ServerKeyExchange message is extended as follows.
enum {
ec_diffie_hellman
} KeyExchangeAlgorithm;
ec_diffie_hellman: Indicates the ServerKeyExchange message contains
an ECDH public key.
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select (KeyExchangeAlgorithm) {
case ec_diffie_hellman:
ServerECDHParams params;
Signature signed_params;
} ServerKeyExchange;
params: Specifies the ECDH public key and associated domain
parameters.
signed_params: A hash of the params, with the signature appropriate
to that hash applied. The private key corresponding to the
certified public key in the server's Certificate message is used
for signing.
enum {
ecdsa(3),
eddsa(TBD5)
} SignatureAlgorithm;
select (SignatureAlgorithm) {
case ecdsa:
digitally-signed struct {
opaque sha_hash[sha_size];
};
case eddsa:
digitally-signed struct {
opaque rawdata[rawdata_size];
};
} Signature;
ServerKeyExchange.signed_params.sha_hash
SHA(ClientHello.random + ServerHello.random +
ServerKeyExchange.params);
ServerKeyExchange.signed_params.rawdata
ClientHello.random + ServerHello.random +
ServerKeyExchange.params;
NOTE: SignatureAlgorithm is "rsa" for the ECDHE_RSA key exchange
algorithm and "anonymous" for ECDH_anon. These cases are defined in
TLS. SignatureAlgorithm is "ecdsa" or "eddsa" for ECDHE_ECDSA.
ECDSA signatures are generated and verified as described in
Section 5.10, and SHA in the above template for sha_hash accordingly
may denote a hash algorithm other than SHA-1. As per ANSI X9.62, an
ECDSA signature consists of a pair of integers, r and s. The
digitally-signed element is encoded as an opaque vector <0..2^16-1>,
the contents of which are the DER encoding corresponding to the
following ASN.1 notation.
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Ecdsa-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
EdDSA signatures are generated and verified according to
[CFRG-EdDSA]. The digitally-signed element is encoded as an opaque
vector<0..2^16-1>, the contents of which is the octet string output
of the EdDSA signing algorithm.
Actions of the sender:
The server selects elliptic curve domain parameters and an ephemeral
ECDH public key corresponding to these parameters according to the
ECKAS-DH1 scheme from IEEE 1363 [IEEE.P1363.1998]. It conveys this
information to the client in the ServerKeyExchange message using the
format defined above.
Actions of the receiver:
The client verifies the signature (when present) and retrieves the
server's elliptic curve domain parameters and ephemeral ECDH public
key from the ServerKeyExchange message. (A possible reason for a
fatal handshake failure is that the client's capabilities for
handling elliptic curves and point formats are exceeded; cf.
Section 5.1.)
5.5. Certificate Request
When this message is sent:
This message is sent when requesting client authentication.
Meaning of this message:
The server uses this message to suggest acceptable client
authentication methods.
Structure of this message:
The TLS CertificateRequest message is extended as follows.
enum {
ecdsa_sign(64),
rsa_fixed_ecdh(65),
ecdsa_fixed_ecdh(66),
(255)
} ClientCertificateType;
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ecdsa_sign, etc. Indicates that the server would like to use the
corresponding client authentication method specified in Section 3.
Actions of the sender:
The server decides which client authentication methods it would like
to use, and conveys this information to the client using the format
defined above.
Actions of the receiver:
The client determines whether it has a suitable certificate for use
with any of the requested methods and whether to proceed with client
authentication.
5.6. Client Certificate
When this message is sent:
This message is sent in response to a CertificateRequest when a
client has a suitable certificate and has decided to proceed with
client authentication. (Note that if the server has used a Supported
Point Formats Extension, a certificate can only be considered
suitable for use with the ECDSA_sign, RSA_fixed_ECDH, and
ECDSA_fixed_ECDH authentication methods if the public key point
specified in it respects the server's choice of point formats. If no
Supported Point Formats Extension has been used, a certificate can
only be considered suitable for use with these authentication methods
if the point is represented in uncompressed point format.)
Meaning of this message:
This message is used to authentically convey the client's static
public key to the server. The following table summarizes what client
certificate types are appropriate for the ECC-based client
authentication mechanisms described in Section 3. ECC public keys
must be encoded in certificates as described in Section 5.9.
NOTE: The client's Certificate message is capable of carrying a chain
of certificates. The restrictions mentioned in Table 4 apply only to
the client's certificate (first in the chain).
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+------------------+------------------------------------------------+
| Client | Client Certificate Type |
| Authentication | |
| Method | |
+------------------+------------------------------------------------+
| ECDSA_sign | Certificate MUST contain an ECDSA- or EdDSA- |
| | capable public key. |
| ECDSA_fixed_ECDH | Certificate MUST contain an ECDH-capable |
| | public key on the same elliptic curve as the |
| | server's long-term ECDH key. |
| RSA_fixed_ECDH | The same as ECDSA_fixed_ECDH. The codepoints |
| | meant different things, but due to changes in |
| | TLS 1.2, both mean the same thing now. |
+------------------+------------------------------------------------+
Table 4: Client Certificate Types
Structure of this message:
Identical to the TLS client Certificate format.
Actions of the sender:
The client constructs an appropriate certificate chain, and conveys
it to the server in the Certificate message.
Actions of the receiver:
The TLS server validates the certificate chain, extracts the client's
public key, and checks that the key type is appropriate for the
client authentication method.
5.7. Client Key Exchange
When this message is sent:
This message is sent in all key exchange algorithms. If client
authentication with ECDSA_fixed_ECDH or RSA_fixed_ECDH is used, this
message is empty. Otherwise, it contains the client's ephemeral ECDH
public key.
Meaning of the message:
This message is used to convey ephemeral data relating to the key
exchange belonging to the client (such as its ephemeral ECDH public
key).
Structure of this message:
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The TLS ClientKeyExchange message is extended as follows.
enum {
implicit,
explicit
} PublicValueEncoding;
implicit, explicit: For ECC cipher suites, this indicates whether
the client's ECDH public key is in the client's certificate
("implicit") or is provided, as an ephemeral ECDH public key, in
the ClientKeyExchange message ("explicit"). (This is "explicit"
in ECC cipher suites except when the client uses the
ECDSA_fixed_ECDH or RSA_fixed_ECDH client authentication
mechanism.)
struct {
select (PublicValueEncoding) {
case implicit: struct { };
case explicit: ECPoint ecdh_Yc;
} ecdh_public;
} ClientECDiffieHellmanPublic;
ecdh_Yc: Contains the client's ephemeral ECDH public key as a byte
string ECPoint.point, which may represent an elliptic curve point
in uncompressed or compressed format. Curves eddsa_ed25519 and
eddsa_ed448 MUST NOT be used here. Here, the format MUST conform
to what the server has requested through a Supported Point Formats
Extension if this extension was used, and MUST be uncompressed if
this extension was not used.
struct {
select (KeyExchangeAlgorithm) {
case ec_diffie_hellman: ClientECDiffieHellmanPublic;
} exchange_keys;
} ClientKeyExchange;
Actions of the sender:
The client selects an ephemeral ECDH public key corresponding to the
parameters it received from the server according to the ECKAS-DH1
scheme from IEEE 1363. It conveys this information to the client in
the ClientKeyExchange message using the format defined above.
Actions of the receiver:
The server retrieves the client's ephemeral ECDH public key from the
ClientKeyExchange message and checks that it is on the same elliptic
curve as the server's ECDH key.
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When this message is sent:
This message is sent when the client sends a client certificate
containing a public key usable for digital signatures, e.g., when the
client is authenticated using the ECDSA_sign mechanism.
Meaning of the message:
This message contains a signature that proves possession of the
private key corresponding to the public key in the client's
Certificate message.
Structure of this message:
The TLS CertificateVerify message and the underlying Signature type
are defined in the TLS base specifications, and the latter is
extended here in Section 5.4. For the ecdsa and eddsa cases, the
signature field in the CertificateVerify message contains an ECDSA or
EdDSA (respectively) signature computed over handshake messages
exchanged so far, exactly similar to CertificateVerify with other
signing algorithms:
CertificateVerify.signature.sha_hash
SHA(handshake_messages);
CertificateVerify.signature.rawdata
handshake_messages;
ECDSA signatures are computed as described in Section 5.10, and SHA
in the above template for sha_hash accordingly may denote a hash
algorithm other than SHA-1. As per ANSI X9.62, an ECDSA signature
consists of a pair of integers, r and s. The digitally-signed
element is encoded as an opaque vector <0..2^16-1>, the contents of
which are the DER encoding [CCITT.X690] corresponding to the
following ASN.1 notation [CCITT.X680].
Ecdsa-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
EdDSA signatures are generated and verified according to
[CFRG-EdDSA]. The digitally-signed element is encoded as an opaque
vector<0..2^16-1>, the contents of which is the octet string output
of the EdDSA signing algorithm.
Actions of the sender:
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The client computes its signature over all handshake messages sent or
received starting at client hello and up to but not including this
message. It uses the private key corresponding to its certified
public key to compute the signature, which is conveyed in the format
defined above.
Actions of the receiver:
The server extracts the client's signature from the CertificateVerify
message, and verifies the signature using the public key it received
in the client's Certificate message.
5.9. Elliptic Curve Certificates
X.509 certificates containing ECC public keys or signed using ECDSA
MUST comply with [RFC3279] or another RFC that replaces or extends
it. X.509 certificates containing ECC public keys or signed using
EdDSA MUST comply with [PKIX-EdDSA]. Clients SHOULD use the elliptic
curve domain parameters recommended in ANSI X9.62, FIPS 186-4, and
SEC 2 [SECG-SEC2] or in [CFRG-EdDSA].
EdDSA keys using Ed25519 and Ed25519ph algorithms MUST use the
eddsa_ed25519 curve, and Ed448 and Ed448ph keys MUST use the
eddsa_ed448 curve. Curves ecdh_x25519, ecdh_x448, eddsa_ed25519 and
eddsa_ed448 MUST NOT be used for ECDSA.
5.10. ECDH, ECDSA, and RSA Computations
All ECDH calculations for the NIST curves (including parameter and
key generation as well as the shared secret calculation) are
performed according to [IEEE.P1363.1998] using the ECKAS-DH1 scheme
with the identity map as key derivation function (KDF), so that the
premaster secret is the x-coordinate of the ECDH shared secret
elliptic curve point represented as an octet string. Note that this
octet string (Z in IEEE 1363 terminology) as output by FE2OSP, the
Field Element to Octet String Conversion Primitive, has constant
length for any given field; leading zeros found in this octet string
MUST NOT be truncated.
(Note that this use of the identity KDF is a technicality. The
complete picture is that ECDH is employed with a non-trivial KDF
because TLS does not directly use the premaster secret for anything
other than for computing the master secret. In TLS 1.0 and 1.1, this
means that the MD5- and SHA-1-based TLS PRF serves as a KDF; in TLS
1.2 the KDF is determined by ciphersuite; it is conceivable that
future TLS versions or new TLS extensions introduced in the future
may vary this computation.)
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An ECDHE key exchange using X25519 (curve ecdh_x25519) goes as
follows: Each party picks a secret key d uniformly at random and
computes the corresponding public key x = X25519(d, G). Parties
exchange their public keys, and compute a shared secret as x_S =
X25519(d, x_peer). If either party obtains all-zeroes x_S, it MUST
abort the handshake (as required by definition of X25519 and X448).
ECDHE for X448 works similarily, replacing X25519 with X448, and
ecdh_x25519 with ecdh_x448. The derived shared secret is used
directly as the premaster secret, which is always exactly 32 bytes
when ECDHE with X25519 is used and 56 bytes when ECDHE with X448 is
used.
All ECDSA computations MUST be performed according to ANSI X9.62 or
its successors. Data to be signed/verified is hashed, and the result
run directly through the ECDSA algorithm with no additional hashing.
The default hash function is SHA-1 [FIPS.180-2], and sha_size (see
Section 5.4 and Section 5.8) is 20. However, an alternative hash
function, such as one of the new SHA hash functions specified in FIPS
180-2 [FIPS.180-2], SHOULD be used instead.
All EdDSA computations MUST be performed according to [CFRG-EdDSA] or
its succesors. Data to be signed/verified is run through the EdDSA
algorithm wih no hashing (EdDSA will internally run the data through
the PH function).
RFC 4492 anticipated the standardization of a mechanism for
specifying the required hash function in the certificate, perhaps in
the parameters field of the subjectPublicKeyInfo. Such
standardization never took place, and as a result, SHA-1 is used in
TLS 1.1 and earlier (except for EdDSA, which uses identity function).
TLS 1.2 added a SignatureAndHashAlgorithm parameter to the
DigitallySigned struct, thus allowing agility in choosing the
signature hash. EdDSA signatures MUST have HashAlgorithm of 0
(None).
All RSA signatures must be generated and verified according to
[PKCS1] block type 1.
5.11. Public Key Validation
With the NIST curves, each party must validate the public key sent by
its peer before performing cryptographic computations with it.
Failing to do so allows attackers to gain information about the
private key, to the point that they may recover the entire private
key in a few requests, if that key is not really ephemeral.
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X25519 was designed in a way that the result of X25519(x, d) will
never reveal information about d, provided it was chosen as
prescribed, for any value of x (the same holds true for X448).
All-zeroes output from X25519 or X448 MUST NOT be used for premaster
secret (as required by definition of X25519 and X448). If the
premaster secret would be all zeroes, the handshake MUST be aborted
(most probably by sending a fatal alert).
Let's define legitimate values of x as the values that can be
obtained as x = X25519(G, d') for some d', and call the other values
illegitimate. The definition of the X25519 function shows that
legitimate values all share the following property: the high-order
bit of the last byte is not set (for X448, any bit can be set).
Since there are some implementation of the X25519 function that
impose this restriction on their input and others that don't,
implementations of X25519 in TLS SHOULD reject public keys when the
high-order bit of the last byte is set (in other words, when the
value of the leftmost byte is greater than 0x7F) in order to prevent
implementation fingerprinting.
Ed25519 and Ed448 internally do public key validation as part of
signature verification.
Other than this recommended check, implementations do not need to
ensure that the public keys they receive are legitimate: this is not
necessary for security with X25519.
6. Cipher Suites
The table below defines new ECC cipher suites that use the key
exchange algorithms specified in Section 2.
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Beyond elliptic curve size, the main issue is elliptic curve
structure. As a general principle, it is more conservative to use
elliptic curves with as little algebraic structure as possible.
Thus, random curves are more conservative than special curves such as
Koblitz curves, and curves over F_p with p random are more
conservative than curves over F_p with p of a special form (and
curves over F_p with p random might be considered more conservative
than curves over F_2^m as there is no choice between multiple fields
of similar size for characteristic 2). Note, however, that algebraic
structure can also lead to implementation efficiencies, and
implementers and users may, therefore, need to balance conservatism
against a need for efficiency. Concrete attacks are known against
only very few special classes of curves, such as supersingular
curves, and these classes are excluded from the ECC standards that
this document references [IEEE.P1363.1998], [ANSI.X9-62.2005].
Another issue is the potential for catastrophic failures when a
single elliptic curve is widely used. In this case, an attack on the
elliptic curve might result in the compromise of a large number of
keys. Again, this concern may need to be balanced against efficiency
and interoperability improvements associated with widely-used curves.
Substantial additional information on elliptic curve choice can be
found in [IEEE.P1363.1998], [ANSI.X9-62.2005], and [FIPS.186-4].
All of the key exchange algorithms defined in this document provide
forward secrecy. Some of the deprecated key exchange algorithms do
not.
8. IANA Considerations
[RFC4492], the predecessor of this document has already defined the
IANA registries for the following:
o Supported Groups Section 5.1
o ECPointFormat Section 5.1
o ECCurveType Section 5.4
For each name space, this document defines the initial value
assignments and defines a range of 256 values (NamedCurve) or eight
values (ECPointFormat and ECCurveType) reserved for Private Use. The
policy for any additional assignments is "Specification Required".
The previous version of this document required IETF review.
NOTE: IANA, please update the registries to reflect the new policy.
NOTE: RFC editor please delete these two notes prior to publication.
IANA, please update these two registries to refer to this document.
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IANA is requested to assign two values from the NamedCurve registry
with names eddsa_ed25519(TBD3) and eddsa_ed448(TBD4) with this
document as reference. IANA has already assigned the value 29 to
ecdh_x25519, and the value 30 to ecdh_x448(TBD2).
IANA is requested to assign one value from SignatureAlgorithm
Registry with name eddsa(TBD5) with this document as reference.
9. Acknowledgements
Most of the text is this document is taken from [RFC4492], the
predecessor of this document. The authors of that document were:
o Simon Blake-Wilson
o Nelson Bolyard
o Vipul Gupta
o Chris Hawk
o Bodo Moeller
In the predecessor document, the authors acknowledged the
contributions of Bill Anderson and Tim Dierks.
10. Version History for This Draft
NOTE TO RFC EDITOR: PLEASE REMOVE THIS SECTION
Changes from draft-ietf-tls-rfc4492bis-03 to draft-nir-tls-rfc4492bis-05:
o Add support for CFRG curves and signatures work.
Changes from draft-ietf-tls-rfc4492bis-01 to draft-nir-tls-rfc4492bis-03:
o Removed unused curves.
o Removed unused point formats (all but uncompressed)
Changes from draft-nir-tls-rfc4492bis-00 and draft-ietf-tls-rfc4492bis-00 to draft-nir-tls-rfc4492bis-01:
o Merged errata
o Removed ECDH_RSA and ECDH_ECDSA
Changes from RFC 4492 to draft-nir-tls-rfc4492bis-00:
o Added TLS 1.2 to references.
o Moved RFC 4492 authors to acknowledgements.
o Removed list of required reading for ECC.
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